1. Field of the Invention
This invention relates to a driving force control system for a four-wheel drive vehicle, which controls driving forces of left and right auxiliary drive wheels distributed from the driving force of main drive wheels connected to an engine by controlling the engagement forces of left and right clutches for the left and right auxiliary drive wheels.
2. Description of the Related Art
Conventionally, a driving force control system of this kind has been proposed by the present assignee in Japanese Laid-Open Patent Publication (Kokai) No. 10-194002. This system is installed on a four-wheel drive vehicle (hereinafter referred to as xe2x80x9cthe vehiclexe2x80x9d) with front wheels as main drive wheels and rear wheels as auxiliary drive wheels. Further, the vehicle includes left and right electromagnetic clutches for connecting and disconnecting the respective left and right rear wheels to and from a propeller shaft, wheel speed sensors for detecting respective wheel speeds of the front and rear wheels, a steering angle sensor for detecting a steering angle of a steering wheel, a yaw rate sensor for detecting a yaw rate of the vehicle, and a lateral acceleration sensor for detecting a lateral acceleration of the vehicle. The driving force control system controls the engagement force of the left and right electromagnetic clutches based upon signals output from these sensors, thereby controlling the torque distributed to the left and right auxiliary drive wheels, i.e., the left and right rear wheels.
More specifically, if there occurs a difference in wheel speed between the front wheels and the rear wheels, i.e., if it is assumed that the front wheels are slipping, the engagement force of the left and right electromagnetic clutches is controlled according to the difference in wheel speed to properly distribute or allocate the torque to the left and right rear wheels, thereby controlling the vehicle to a four-wheel drive mode. Further, when it is detected that the vehicle is cornering with an oversteering tendency or with an understeering tendency, the engagement forces of the respective left and right clutches are decreased or increased independently of each other depending on the cornering conditions of the vehicle to thereby control the torque distributed to the left and right rear wheels such that the oversteering tendency or the understeering tendency can be eliminated.
According to the conventional driving force control system, when the vehicle corners or turns at a relatively low vehicle speed on a road surface having a high frictional resistance, with large engagement forces of the left and right electromagnetic clutches, so-called tight-turn braking can act on the vehicle due to the difference in wheel speed between the left and right rear wheels and the surface resistance of the road. When such a tight-turn braking phenomenon occurs, the cornering characteristics of the vehicle are degraded. The tight-turn braking phenomenon can be overcome by the same control method as mentioned above which is used in eliminating the oversteering or understeering tendency. However, this method necessitates the use of many sensors enumerated above, and results in an increase in manufacturing cost. Further, according to this method, the driving forces transmitted to the rear wheels are increased only after an actual slip of the front wheels occurs, which means that the response of slippage-eliminating control is not high.
It is an object of the present invention to provide a driving force control system for a four-wheel drive vehicle, which is capable of preventing occurrence of a tight-turn braking phenomenon by relatively simple construction to thereby reduce the manufacturing costs thereof, and at the same time, enhancing the response of slippage-eliminating control on main drive wheels and stability of the vehicle.
To attain the above object, the present invention provides a driving force control system for a four-wheel drive control vehicle including a pair of front wheels, a pair of rear wheels, an engine, a transmission for connecting one of the pair of front wheels and the pair of rear wheels to the engine, as main drive wheels, and left and right clutches for connecting another of the pair of front wheels and the pair of rear wheels to the main drive wheels, as left and right auxiliary drive wheels, respectively, the driving force control system controlling the engagement force of each of the left and right clutches to thereby control distribution of a driving force of the main drive wheels to the auxiliary left and right wheels.
The driving force control system according to the invention comprises:
wheel speed detection means for detecting wheel speeds indicative of respective rotational speeds of the front and rear wheels;
average wheel speed calculation means for calculating an average wheel speed of the main drive wheels and an average wheel speed of the auxiliary drive wheels, based on the detected wheel speeds;
first index calculation means for calculating a first index representative of a relative relationship in magnitude between the average wheel speed of the main drive wheels and the average wheel speed of the auxiliary drive wheels;
second index calculation means for calculating a second index representative of a relative relationship in magnitude between respective wheel speeds of the left and right auxiliary wheels; and
clutch engagement force determination means for determining respective engagement forces of the left and right clutches, depending on a relative relationship in magnitude between the first index and the second index.
According to this driving force control system, the average wheel speed calculation means calculates an average wheel speed of the main drive wheels and an average wheel speed of the auxiliary drive wheels, based on the detected wheel speeds. Further, a first index representative of a relative relationship in magnitude between these average wheel speeds is calculated, and a second index representative of a relative relationship in magnitude between wheel speeds of the left and right auxiliary wheels is calculated. Then, the respective engagement forces of the left and right clutches are determined in dependence on a relative relationship in magnitude between the first index and the second index. Thus, the engagement forces of the left and right electromagnetic clutches are determined such that the resulting engagement forces reflect not only the relative relationship in magnitude between the average wheel speed of the main drive wheels and that of the auxiliary drive wheels but also the relative relationship in magnitude between the wheel speeds of the respective left and right auxiliary drive wheels. Therefore, even when a tight turn braking force acts on the vehicle when it is cornering at a relatively low speed on a road surface having a high frictional resistance, e.g., on a paved road surface, the engagement forces of the left and right clutches can be controlled such that the inner one of the rear wheels is inhibited from rotating, by the braking force, while the other is allowed to rotate by slip of the associated clutch. As a result, it is possible to prevent occurrence of tight turn braking. As described above, such advantageous effects as mentioned above can be obtained by omitting the use of sensors conventionally used, such as a steering angle sensor, a yaw rate sensor, and a lateral acceleration sensor, and instead using the wheel speed detection means alone, whereby manufacturing costs of the vehicle can be reduced.
Preferably, the driving force control system further includes average wheel acceleration-calculating means for calculating an average wheel acceleration of the main drive wheels and an average wheel acceleration of the auxiliary drive wheels based on the detected wheel speeds, and increasing correction means for increasing the engagement forces of the left and right clutches, when the calculated average wheel acceleration of the main drive wheels is larger than the calculated average wheel acceleration of the auxiliary drive wheels.
According to this preferred embodiment, when the calculated average wheel acceleration of the main drive wheels is larger than the calculated average wheel acceleration of the auxiliary drive wheels, i.e., when the main drive wheels are slipping, the engagement forces of the left and right clutches are increased. That is, the slippage-eliminating control of the main drive wheels is carried out depending on the relative relationship in magnitude between the acceleration of the main drive wheels and that of the auxiliary drive wheels. Because the relative relationship in magnitude between the acceleration of the main drive wheels and that of the auxiliary drive wheels represents a slipping condition of the main drive wheels on more of a real time basis than the difference between the wheel speed of the main drive wheels and that of the rear drive wheels, the response of the slippage-eliminating control can be enhanced compared with a case in which this control is started upon occurrence of a difference in wheel speed between the main drive wheels sand auxiliary drive wheels.
More preferably, the increasing correction means increases the engagement forces of the left and right clutches according to an extent by which the calculated average wheel acceleration of the main drive wheels exceeds the calculated average wheel acceleration of the auxiliary drive wheels.
Preferably, the driving force control system further includes vehicle speed calculation means for calculating a vehicle speed of the four-wheel drive vehicle based on the detected wheel speeds, and decreasing correction means for decreasing the engagement forces of the left and right clutches as the calculated vehicle speed increases.
According to this preferred embodiment, the engagement forces of the left and right clutches are decreased as the calculated vehicle speed increases, i.e., as the slippage of the main drive wheels is considered to be smaller. Therefore, as the vehicle speed increases, the driving force distributed to the auxiliary drive wheels is made smaller. In other words, as less amount of slip is decreased, the driving force of the auxiliary drive wheels is required. In accordance with this, the preferred embodiment decreases the driving force of the auxiliary drive wheels so that the driving status of the vehicle is made closer to the two-wheel drive mode. This allows the reduction of size of the overall driving system including the clutches. Further, because the driving status of the vehicle is made closer to the two-wheel drive mode as the vehicle speed increases, so that the resistance to rotations of the elements of the driving system can be made smaller, whereby the fuel economy can be improved.
Preferably, the driving force control system further includes vehicle acceleration-calculating means for calculating a vehicle acceleration of the four-wheel drive vehicle based on the driving force of the main drive wheels, and second decreasing correction means for decreasing the engagement forces of the left and right clutches as the calculated vehicle acceleration decreases.
According to this driving force control system, the vehicle acceleration is calculated based on the driving force demanded by the main drive wheels, and the engagement forces of the left and right clutches are decreased as the vehicle acceleration decreases. This makes it possible to decrease the engagement forces of the clutches when the acceleration is not required, e.g., in the case of the accelerator pedal having been released. Further, because the engagement forces of the clutches are decreased for correction based on the vehicle acceleration, differently from the prior art in which the engagement forces are calculated based on the vehicle speed, it is possible to obtain appropriate engagement forces commensurate with the acceleration of the vehicle. As a result, when the road surface resistance is small and the accelerator is not stepped on, it is possible to prevent the engagement forces of the left and right engagement forces from becoming larger than required by the acceleration of the vehicle. This makes it possible to prevent the auxiliary drive wheels from undergoing a slip.
More preferably, the drive force control system includes gear position detection means for detecting a gear position of the transmission, and the second decreasing correction means decreases the engagement forces of the left and right clutches as the detected gear position is one for a higher speed.